If you pull the intake off the motor you will see those letters stamped on the block in the lifter valley. Don't make the mistake of just picking up any 5.0 roller block. Get an '87-'92 motor out of a 5.0 mustang or lincoln LSC; same exact thing. That way you get forged pistons, a decent stock cam, and decent factory heads to start with.
A lot of the lincoln LSC's end up in junkyards because of tranny problems. A lot of the engines in them (if maintained) still have great compression (because they generally weren't beat on like stangs) and you can sometimes get away with using the shortblock as is.
Three x's (XXX) or three y's (YYY); both letters indicate that you have a roller block.
Oh, and unless the motor in your '84 was replaced, it's not a roller block. The first roller blocks were put in '85's.
ok here is a real easy way to tell. when you have the intake off look and see if the top is machined on each hole for the lifters. if it's machined it's a roller block. if it's not machined then it's not